In a typical refrigeration system, refrigerant circulates continuously through a closed circuit. The term "circuit", as used herein, refers to a physical apparatus whereas the term "cycle" as used herein refers to operation of a circuit, e.g., refrigerant cycles in a refrigeration circuit. The term "refrigerant", as used herein, refers to refrigerant in liquid, vapor and/or gas form. Components of the closed circuit cause the refrigerant to undergo temperature/pressure changes which result in energy transfer. Typical components of a refrigeration system include, for example, compressors, condensers, evaporators, control valves, and connecting piping.
Energy efficiency is an important factor in the assessment of refrigeration systems. Increased energy efficiency is typically achieved by utilizing more expensive and more efficient components, by adding extra insulation adjacent to the area to be refrigerated, or by other costly additions. Increasing the energy efficiency of a refrigeration system therefore usually results in an increase in the cost of the system. It is, therefore, desirable to increase the efficiency of a refrigeration system and minimize any increase in the cost of the system.
In some apparatus utilizing refrigeration systems, more than one area needs to be refrigerated, and at least one area requires more refrigeration than another area. A typical household refrigerator, which includes a freezer compartment and a fresh food compartment, is one example of such an apparatus. The freezer compartment is preferably maintained between about -25.degree. and about -10.degree. C., and the fresh food compartment between about +1.degree. and about +8.degree. C.
To meet these temperature requirements, a typical refrigeration system includes a compressor coupled to an evaporator. The terms "coupled" and "connected" are used herein interchangeably. When two components are coupled or connected, this means that the components are linked, directly or indirectly in some manner in refrigerant flow relationship, even though another component or components may be positioned between them.
Referring again to the refrigeration system for a typical household refrigerator, the evaporator is maintained at about -25.degree. C. (an actual range of about -15.degree. to -35.degree. C. is typically used) and air is blown across the coils of the evaporator. The flow of the evaporator-cooled air is controlled, for example, by barriers. A first portion of the evaporator-cooled air is directed to the freezer compartment and a second portion to the fresh food compartment.
To cool a fresh food compartment, it is also possible to utilize an evaporator operating at, for example, about -5.degree. C. (or in a range from about -10.degree. C. to about 0.degree. C.). A typical refrigeration system utilized in household refrigerators, therefore, produces its refrigeration effect by operating the evaporator at a temperature which is appropriate for the freezer compartment but lower than it needs to be for the fresh food compartment. A typical household refrigerator therefore uses more energy to cool the fresh food compartment than is necessary, operating at reduced energy efficiency.
This household refrigerator example is provided for illustrative purposes only. Many apparatus other than household refrigerators utilize refrigeration systems which include an evaporator operating at an unnecessarily low temperature.
A refrigeration system which operates at reduced energy consumption is described in U.S. Pat. No. 5,156,016. It utilizes at least two evaporators and a plurality of compressors or a compressor having a plurality of stages. This device utilizes the pressure difference between the high pressure and the low pressure refrigerant to operate a switching valve having bellows therein. However, for refrigeration systems that operate at 25 kg./cm..sup.2 and are required to be functional without rupture at 100 kg./cm..sup.2 or greater, the cost of such a valve is quite high. A need exists for a less expensive and equally efficient refrigeration system to function under such conditions.